1. Field of the Invention
The invention relates to compositions and compounds that have large two-photon or higher-order absorptivities, which, after excitation, generate Lewis or Brønsted acids, radicals or a combination thereof The invention also relates to methods of making and using the compositions and compounds.
2. Discussion of the Background
Two-photon or higher-order absorption refers to the initial simultaneous absorption of two or more photons (also referred to as multi-photon absorption) without the actual population of an excited state by the absorption of a single photon.
Molecular two-photon absorption was predicted in Göppert-Mayer, M., Ann. Phys. 1931, 9, 273. Upon the invention of pulsed ruby lasers in 1960, experimental observation of two-photon absorption became reality. In the years since, multi-photon excitation has found application in biology and optical data storage, as well as in other fields.
Although interest in multi-photon excitation has exploded, there is a paucity of two-photon absorbing dyes with adequately strong two-photon absorption in the correct spectral region for many applications.
There are two key advantages of two-photon (or higher-order) induced processes relative to single-photon induced processes. Whereas single-photon absorption scales linearly with the intensity of the incident radiation, two-photon absorption scales quadratically. Higher-order absorptions will scale with yet a higher power of incident intensity. As a result, it is possible to perform multi-photon induced processes with three dimensional spatial resolution. Further, because these processes involve the simultaneous absorption of two or more photons, the chromophore is excited with a number of photons whose total energy equals the energy of a multi-photon absorption transition, although each photon individually has insufficient energy to excite the chromophore. Because the exciting light is not attenuated by single-photon absorption in this case, it is possible to excite selectively molecules at a greater depth within a material than would be possible via single-photon excitation by use of a beam that is focused to that depth in the material. These two advantages also apply to, for example, excitation within tissue or other biological materials. In multi-photon lithography or stereolithography, the nonlinear scaling of absorption with intensity can lead to the ability to write features of a size below the diffraction limit of light, and the ability to write features in three dimensions, which is also of interest for holography.
The ability to realize many of the possible applications of two-photon or higher-order absorption by molecules rests on the availability of chromophores with large two-photon or higher-order absorption cross sections. We have taught in U.S. Pat. No. 6,267,913, which is incorporated herein by reference, that certain classes of molecules exhibit enhanced two-photon or multi-photon absorptivities. These molecules can be categorized as follows:                a) molecules in which two donors are connected to a conjugated π-electron bridge (abbreviated “D-π-D” motif);        b) molecules in which two donors are connected to a conjugated π-electron bridge which is substituted with one or more electron accepting groups (abbreviated “D-A-D” motif);        c) molecules in which two acceptors are connected to a conjugated π-electron bridge (abbreviated “A-π-A” motif); and        d) molecules in which two acceptors are connected to a conjugated π-electron bridge which is substituted with one or more electron donating groups (abbreviated “A-D-A” motif).        
Accordingly, molecules from the aforementioned classes can be excited efficiently by simultaneous two-photon (or higher-order) absorption, leading to efficient generation of electronically excited states. These excited state species can be exploited in a great variety of chemical and physical processes, with the advantages enabled by multiphoton excitation. For example, by employing polymerizable resin formulations containing cross-linkable acrylate containing monomers and D-π-D molecules as two-photon initiators of radical polymerization, complex three-dimensional objects can be prepared using patterned two-photon excitation. Most two-photon induced photopolymerization processes involve radical reactions in which there is some volume decrease upon polymerization (Cumpston et al. Nature 398, (1999) 51; Belfield, K. D. et al. J. Am. Chem. Soc. 122, (2000) 1217).
The applications that depend upon two-photon or multi-photon excitation also require that the two-photon or multiphoton excited states cause a chemical or physical change in the exposed region of the materials. Such changes can result from the generation of a Brønsted or Lewis acid and/or radical species and subsequent further reactions of that species with other components in the material, for example, resulting in cleavage of a functional group from a polymer or initiation of a polymerization, as is well known to one skilled in the art of lithography.
Under one photon excitation conditions it has been shown that sulfonium and iodonium salts are effective for the generation of Brønsted acids. Methods for the synthesis of sulfonium salts are well documented in J. L. Dektar and N. P. Hacker, “Photochemistry of Triarylsulfonium Salts”, J. Am. Chem. Soc. 112, (1990) 6004-6015 which are incorporated herein by reference. Additional methods for synthesizing onium salts of the the general type described in the invention can be prepared conveniently from aryl aliphatic sulfides and primary aliphatic halides or benzyl halides, by well known methods such as those described in Lowe, P. A., “Synthesis of Sulfonium Salts”, The Chemistry of the Sulfonium Group (Part 1), ed. C. J. M. Sterling, John Wiley & Sons, Ltd., (1981), p 267 et seq and as described in U.S. Pat. Nos. 5,302,757, 5,274,148, 5,446,172, 5,012,001, 4,882,201, 5,591011, and 2,807,648, which are all incorporated herein by reference. Methods for the synthesis of iodonium salts are well documented in C. Herzig and S. Scheiding, DE 4,142,327, CA 119,250,162 and C. Herzig, EP 4,219,376, CA 120,298,975 and U.S. Pat. Nos. 5,079,378, 4,992,571, 4,450,360, 4,399,071, 4,310,469, 4,151,175, 3,981,897, and 5,144,051 which are incorporated herein by reference.
It is known to those skilled in the art that epoxide-containing monomers exhibit relatively small shrinkage upon polymerization. It is also known that expoxide monomers as well as others, such as vinyl ether monomers, can be photo-polymerized under one photon excitation conditions using iodonium salts and sulfonium salts as photoacid generating initiators as described by: Crivello, J. V.; Lam, J. H. W. Macromolecules, 1977, 10, 1307; DeVoe, R. J.; Sahyn, M. R. V.; Schmidt, E. Can. J. Chem. 1988, 66, 319; Crivello, J. V.; Lee, J. J. Polym. Sci. Polym. Chem. Ed. 1989, 27 3951; Dektar, J.; Hacker, N. P. J. Am. Chem. Soc. 1990, 112, 6004; Crivello, J. V.; Lam, J. H. W.; Volante, C. N. J. Rad. Curing, 1977, 4, 2; Pappas, S. P.; Pappas, B. C.; Gatechair, L. R.; Jilek, J. H. Polym. Photochem. 1984, 5, 1; Welsh, K. M.; Dektar, J. L.; Garcia-Garibay, M. A.; Hacker, N. P.; Turro, N. J. J. Org. Chem. 1992, 57, 4179; Crivello, J. V.; Kong, S. Macromolecules, 2000, 33, 825, which are incorporated herein by reference.
It is known that dialkyl aryl sulfonium ions—as described by Saeva, F. D.; Morgan, B. P. J. Am. Chem. Soc., 1984, 106, 4121; Saeva, F. D. Advances in Electron Transfer Chem. 1994, 4, 1, which are incorporated herein by reference—and iodonium salts can be sensitized through electron transfer by the addition of other molecules. These include Class I and Class II photoacid generating species, as described by Saeva et al. (cited above).